Scientists Discover Two New Interstellar Molecules:
Point to Probable Pathways for Chemical Evolution in Space

A team of scientists using the National Science Foundation's
Robert C. Byrd Green Bank Telescope (GBT) has discovered
two new molecules in an interstellar cloud near
the center of the Milky Way Galaxy. This discovery
is the GBT's first detection of new molecules,
and is already helping astronomers better understand the
complex processes by which large molecules form in space.

Diagram of the 10-atom molecule propanal
and the 8-atom molecule propenal..

CREDIT: NRAO/AUI/NSF

The 8-atom molecule propenal and the 10-atom molecule
propanal were detected in a large cloud of gas
and dust some 26,000 light-years away in an area
known as Sagittarius B2. Such clouds, often many
light-years across, are the raw material from which new stars
are formed.

"Though very rarefied by Earth standards, these interstellar
clouds are the sites of complex chemical reactions that occur
over hundreds-of-thousands or millions of years," said Jan
M. Hollis of the NASA Goddard Space Flight Center in Greenbelt,
Md. "Over time, more and more complex molecules can be
formed in these clouds. At present, however, there is no
accepted theory addressing how interstellar molecules containing
more than 5 atoms are formed."

So far, about 130 different molecules have been discovered
in interstellar clouds. Most of these molecules contain
a small number of atoms, and only a few molecules with eight
or more atoms have been found in interstellar clouds. Each
time a new molecule is discovered, it helps to constrain the
formation chemistry and the nature of interstellar dust grains,
which are believed to be the formation sites of most complex
interstellar molecules.

Hollis collaborated with Anthony Remijan, also of NASA Goddard;
Frank J. Lovas of the National Institute of Standards
and Technology in Gaithersburg, Md.; Harald Mollendal of the
University of Oslo, Norway; and Philip R. Jewell
of the National Radio Astronomy Observatory (NRAO) in Green
Bank, W.Va. Their results were accepted for publication in
the Astrophysical Journal Letters.

In the GBT experiment, three aldehyde molecules were
observed and appear to be related by simple hydrogen
addition reactions, which probably occur on the surface of
interstellar grains. An aldehyde is a molecule that contains
the aldehyde group (CHO): a carbon atom singly bonded to a
hydrogen atom and double-bonded to an oxygen atom; the
remaining bond on that same carbon atom bonds to the
rest of the molecule.

Starting with previously reported propynal (HC2CHO),
propenal (CH2CHCHO) is formed by adding two hydrogen atoms.
By the same process propanal (CH3CH2CHO) is formed from propenal.

After these molecules are formed on interstellar dust grains,
they may be ejected as a diffuse gas. If enough molecules
accumulate in the gas, they can be detected with a radio
telescope. As the molecules rotate end-for-end, they
change from one rotational energy state to another,
emitting radio waves at precise frequencies. The "family"
of radio frequencies emitted by a particular molecule
forms a unique "fingerprint" that scientists can use
to identify that molecule. The scientists identified
the two new aldehydes by detecting a number of
frequencies of radio emission in what is termed
the K-band region (18 to 26 GHz) of the electromagnetic spectrum.

"Interstellar molecules are identified by means of the
frequencies that are unique to the rotational spectrum
of each molecule," said Lovas. "These are either
directly measured in the laboratory or calculated from
the measured data. In this case we used the calculated
spectral frequencies based on an analysis of the literature data."

Complex molecules in space are of interest for many reasons,
including their possible connection to the formation
of biologically significant molecules on the early Earth.
Complex molecules might have formed on the early Earth,
or they might have first formed in interstellar clouds
and been transported to the surface of the Earth.

Molecules with the aldehyde group are particularly
interesting since several biologically significant
molecules, including a family of sugar molecules, are aldehydes.

"The GBT can be used to fully explore the possibility
that a significant amount of prebiotic chemistry
may occur in space long before it occurs on a newly
formed planet," said Remijan. "Comets form from
interstellar clouds and incessantly bombard a newly
formed planet early in its history. Craters on our
Moon attest to this. Thus, comets may be the
delivery vehicles for organic molecules necessary
for life to begin on a new planet."

Laboratory experiments also demonstrate that
atomic addition reactions -- similar to those
assumed to occur in interstellar clouds -- play
a role in synthesizing complex molecules by
subjecting ices containing simpler molecules such
as water, carbon dioxide, and methanol to ionizing
radiation dosages. Thus, laboratory experiments
can now be devised with various ice components
to attempt production of the aldehydes observed with the GBT.

"The detection of the two new aldehydes,
which are related by a common chemical pathway
called hydrogen addition, demonstrates that
evolution to more complex species occurs routinely
in interstellar clouds and that a relatively simple
mechanism may build large molecules out of smaller ones.
The GBT is now a key instrument in exploring
chemical evolution in space," said Hollis.

The GBT is the world's largest fully steerable radio telescope;
it is operated by the NRAO.

"The large diameter and high precision of the GBT
allowed us to study small interstellar clouds that
can absorb the radiation from a bright, background
source. The sensitivity and flexibility of the
telescope gave us an important new tool for the study
of complex interstellar molecules," said Jewell.